ANNULOSE DIFFERENTIATION. 51 



omy and development compel the morphologist to regard 

 the whole of the Metazoa as modifications of one actual or 

 ideal primitive type, which is a sac with a double cellular 

 wall, inclosing a central cavity and open at one end. This is 

 what Haeckel terms a Gastrcea. The inner wall of the sac is 

 the hypoblast {endoclerm of the adult), the outer the epiblast 

 (ectoder?n). Between the two, in all but the very lowest 

 Metazoa, a third layer, the mesoblast {mesoderm of the adult), 

 makes its appearance. 



In the Porifera, the terminal aperture of the gastrasa 

 becomes the egestive opening of the adult animal, and the 

 ingestive apertures are numerous secondary pore-like aper- 

 tures formed by the separation of adjacent cells of the ec- 

 toderm and endoderm. The body may become variously 

 branched, a fibrous or spicular endoskeleton is usually de- 

 veloped in the ectoderm, and no perivisceral cavity is de- 

 veloped. There are no appendages for locomotion or pre- 

 hension ; no nervous system nor sensory organs are known to 

 exist ; nor are there any circulatory, respiratory, renal, or 

 generative organs. 



In the Coelenterata, the terminal aperture of the gastra?a 

 becomes the mouth, and, if pores perforate the body-walls, 

 they do not subserve the ingestion of food. There is no sep- 

 arate perivisceral cavity, but, in many, an enteroccele or sys- 

 tem of cavities, continuous with, but more or less separate 

 from, the digestive cavity, extends through the body. Pre- 

 hensile appendages, tentacula, are developed in great variety. 

 A chitinous exoskeleton appears in some, a calcareous or chit- 

 inous endoskeleton in others. There are no circulatory, re- 

 spiratory, or renal organs (though it is possible that certain 

 cells in the Porpltte, e. g., may have a uropoietic function); 

 but special genital organs make their appearance, as do a 

 definitely-arranged nervous system and organs of sense. 



The lowest Turbellarla are on nearly the same grade of 

 organization as the lower Ccelenterata, but the thick meso- 

 derm is traversed by canals which constitute a water-vascular 

 system. In the adult state these canals open, on the one side, 

 into the interstices of the mesodermal tissues, and, on the 

 other, communicate with the exterior. Their analogy to the 

 contractile vacuoles of the Infusoria on the one hand, and to 

 the segmental organs of the Annelids on the other, lead me 

 to think that they are formed by a splitting of the mesoblast, 

 and that they thus represent that form of perivisceral cavity 

 which I have termed a schizocoele. A nervous system, con- 



